Seems simple enough. But when it comes to a triathlon, those babies can be the difference between reaching your goal time and sliding in late. If you have to tie your shoes, have trouble getting them on your feet, if they’re too heavy—that all slows you down, says Karen Smyers, four-time World Champion triathlete.

Here’s what to consider before you invest a Benjamin into a pair of running kicks.

Your Experience LevelIn general, lighter shoes help you run faster—but don’t just hop into a pair of racing flats. “The better the athlete, the lighter the shoe,” says Timothy Noakes, M.D., a sports science researcher at the University of Cape Town. Why? The best athletes run on the front of their feet—they don’t need heel cushioning, he adds. (Check out this article on the advantages of forefront running.)

Aside from the form difference, well-trained racers are also fit enough to take the beating of a high-intensity race on a lighter shoe, says Sharone Aharon, M.S., and USAT Level 3 coach. A lighter shoe has a thinner outer sole, so you’ll rely more on your body’s natural ability to absorb shock than running with a cushioned shoe.

If you’re running heel-to-toe, additional cushioning on a shoe helps absorb more shock, Noakes says. You can tell if you are a heel-striker by looking at your current shoes. Are they run down on the back? That means you’re striking there first.

Cushioned shoes also tend to have thicker portions in the arch to prevent your feet from rolling inward, says Aharon. “But that adds weight to the shoe,” he warns.

Pro or beginner, remember this: About 15 miles of running will break your legs down a bit more than shorter runs, says Smyers. It’s a good number to start considering additional cushioning.

ComfortThe number one rule of shoes? They must be comfortable, says Aharon—who’s bought all of the latest and greatest only to find he can’t run in them without his own insoles. The good news: Most running stores let you run around the block to test a pair. While this is better than buying shoes on a whim, Aharon urges that it takes more than a quick jog to see if a shoe is a good fit.

SizeYou want a shoe to be a little on the big side—your leg moves around within shoe, and your feet could be swollen from the bike ride at the beginning of your run. Go running shoe shopping at the end of the day—your feet are swollen and you won’t buy shoes that are too small. “If you buy a shoe that is the same size as your walking shoe, you’ll end up with black nails after the race,” he adds. No thank you.

Hot SpotsRight when you put a shoe on, you’ll be able to find its hot spots, says Aharon. Is it too tight on the top? Is your baby toe getting squashed from side seams? Some hot spots can be fixed, he says, but others—like the squashed baby toe—are signs of a poorly fitting shoe. If a shoe feels tight at the top, but otherwise is great, consider elastic laces like Yankz that’ll loosen it and save you transition time. “For all the money you can spend in a triathlon, the most important and cheapest thing you can do is buy a pair of elastic laces,” says Smyers. They’ll make slipping a shoe on much easier, and most running stores have them for no more than $10.

Drainage HolesNo one wants that soggy feet feeling—especially if you’re racing. “I’ve done Ironmans where I’ve felt my shoe accumulating water. It makes it really heavy,” Smyers says. The solution is simple: Find running shoes that have drainage holes and breathable tops to keep you cool and dry after the swim. Hot, black pavement can sometimes be 10 degrees warmer than air temperature.

Go Without SocksSmyers doesn’t usually wear socks when she competes. In fact, many triathletes don’t. “It saves you time. Good shoe companies are aware that people aren’t going to be wearing socks. They’ll make sure the seams are more flush.”

Pay attention to companies that market to triathletes. Are the pros wearing a certain brand? Does the brand have a long-standing history with the sport? That’s a good way to tell if it’s a keeper, says Smyers.

Make sure to train without socks—once a week or just for your track workouts—if you’re planning on doing it for the race, she says. “As long as it’s not painful or turns to a callous, and you feel good running, your feet will probably get used to them.”

To help avoid blisters, Smyers recommends baby powder or Body Glide. It rolls onto the actual shoe or your foot and protects problem areas from wear and tear.

Look for the LoopWhat’s the little loop on the back of some shoes for? It helps you hold the shoe when you’re sliding your foot into it, says Smyers. It’s a time saver in the bike to run transition. Smyers recommends Saucony’s Grid Type A4. (Saucony.com; $95.00)

Cassie ShortsleeveFreelance WriterCassie Shortsleeve is a skilled freelance writer and editor with almost a decade of experience reporting on all things health, fitness, and travel.

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